The aim of this research is to understand the controls governing the proliferation and differentiation of a multipotent stem cell, the interstitial cell (I-cell) of hydra. The approach is to construct tissue with different cell compositions to determine which cell types influence the I-cell to underto proliferation or a particular differentiation, and thereby elucidate the pattern of cell-cell interactions responsible for I-cell behavior. Understanding the position-dependent effects on I-cell differentiation is of particular interest. The cell composition of the tissue will be altered by selective deletion of cell types in intact animals, by grafting pieces of the animal to one another, and with a cell aggregation technique. In the last, cell suspensions of selected cell compositions are made and aggregates formed which develop into normal animals. The technique permits the analysis of the behavior of individual populations of I-cells in tissues consisting of a wide variety of cell compositions. The patterns of I-cell behavior will be analyzed quantitatively by measuring changes in population sizes of I-cells and differentiation products with methods developed for quantiating all hydra cell labelled cohorts of I-cells by combining the methods for quantitating cell populations with autoradiography. With this technology, the I-cell system (I-cells and 5 differentiation products) has been extensively characterized and some of the elements of control elucidated. The specific aims of this proposal are: 1) to extend the characterization of the system by examining the complexity of the I-cell system and 2) to study the basis of position-dependent differentiation; 3) to determine the cell composition; 4) to examine aspects of the commitment process an I-cell undergoes controlling the differentiation of multipotent cells in basic to an understanding of some classes of defects that occur during human development.